Polymeric thick film inks consist of particles containing conductive materials dispersed in an organic vehicle or medium containing volatile solvent and a polymeric resin. After screen-printing the composition is dried typically by heating at temperatures of up to 150.degree. C. where the organic solvent is dried-off.
The conductive material is responsible for imparting to the thick film material the desired level of conductivity. This is normally limited to more than 0.01 Ohm/sq/mil. The conductive particles consist of typically silver metal for high conductivity and good resistance to oxidation and can be found in flake and/or non-flake morphologies.
After drying, the polymeric resin's primary function is to bind together the conductive particles to form an electrically conductive circuit pattern. Additionally, the binder system is required to impart the necessary adhesion to the desired substrate. In the case of flexible substrates which may or may not be surface treated, thermoplastic binder systems are typically employed. In general, these consist of polyesters, acrylics, vinyl or polyurethane polymers and may be combined to obtain optimum properties.
Furthermore, the resinous component is also responsible for providing the conductor composition with the required surface hardness, resistance to environmental changes and flexibility.
Historically, linear polyester, acrylic or vinyl copolymer-based resins have been typically used as the polymeric binder for general purpose membrane switch conductor pastes, having reasonably good abrasion resistance, chemical resistance, heat stability and flexibility. Other types of resins such as polyhydroxyethers (UCAR Phenoxy Resins) have also been utilized for higher degrees of abrasion resistance though normally higher drying temperatures are required (140.degree.-150.degree. C.). Polyurethane resins have also been utilized where surface hardness is not as important but a high degree of flexibility is needed on difficult surfaces such as rubber or polyethylene which is very smooth.
Thermoset polymeric compositions such as epoxy or phenolic resin have been commonly employed in polymeric thick film inks for rigid substrates. For these applications cross-linking polymers are preferred to ensure a high degree of hardness/abrasion resistance and heat stability (e.g., during soldering) on, for example, printed circuit boards.
The solvent system is required to primarily dissolve the polymeric resin system as well as adequately wet the substrate during the screen-printing operation. Additives may also be employed to fine-tune viscosity for good screen printing characteristics enabling accurate and reproducible production, or to modify the binder system acting as, for example, a plasticizer to enhance flex properties.
Present and future technology required for membrane touch switches is focused on 1) providing lower thick film raw material costs (reduced conductive metal loadings per unit conductivity), 2) lower substrate costs, 3) reducing operating costs (i.e., higher throughput capability), 4) increased durability to harsh environmental changes, 5) reliable usage at high working temperatures (e.g., up to 105.degree. C.), 6) increased surface hardness for resistance to repeated high pressure electrical connector actuations, and 7) adequate conductor flexibility in order to ensure conductive stability after both the creasing and folding operations encountered during construction as well as repeated stretching and bending deformations during the life of a switch.
Prior art attempts at such compositions, such as U.S. Pat. No. 4,595,605 discloses an invention relating to conductive compositions which are solderable and flexible and that can be bonded directly to substrates. These compositions are made up from silver flake and vinyl chloride/vinyl acetate copolymer. Once cured the compositions demonstrate good adhesion, solderability and flexibility characteristics. Additionally, these compositions are capable of being soldered by non-silver bearing solder.
Other attempts at compositions are described in U.S. Pat. No. 5,089,173 which discloses an invention relating to a conductive composition comprising at least one thermoplastic vinyl chloride/vinyl acetate/dicarboxylic acid multipolymer resin; a second thermoplastic resin selected from the group consisting of at least one thermoplastic urethane resin; at least one thermoplastic polyester resin; or at least one thermoplastic polyurethane and at least one thermoplastic polyester resin; a tertiary amine; and an effective amount of at least one organic solvent and silver flake.
When the mixture of multipolymer resin or resins with the polyurethane or polyester resin or resins are thermally cured, it is believed that the unexpected high conductivity of the cured product results from a reaction of tertiary amine with the acid groups in the multipolymer, causing the formation of ionic groups in the cured resin system. The presence of these ionic groups are believed to cause the desired conductivity.
The present invention differs in that the terpolymer consists of vinyl chloride/vinyl acetate/vinyl alcohol or modified vinyl alcohol and gives more stable compositions.
The present invention relates primarily to the polymeric component of the conductor composition to provide enhanced properties in terms of those cited in 6) and 7) hereinabove which would be perceived as performance benefits for the membrane switch manufacturer. In this respect, the vinyl alcohol or hydroxyalkyl acrylate components of the vinyl terpolymer system of the present invention are thought to be playing a beneficial role, especially on non-print treated substrates. In the second instance, the invention relates to providing possibilities for cost reduction for the membrane switch manufacturer through use of conductor pastes with reduced metal loadings. This invention extends itself beyond silver and includes carbon/graphite/silver and carbon/graphite particles for conductors which reduce costs. In the third instance, the invention relates to providing possibilities for the membrane switch producer to reduce raw material cost through use of cheaper substrates such as non-print treated polyester.
The invention relates to a highly durable and flexible screen-printable conductor composition which is useful for making membrane touch switches and shielding applications. The invention is primarily directed at applications requiring high degrees of elasticity to withstand deformational stresses involved during repeated keyboard pad attuations. In particular, the carbon/graphite composition has typically higher abrasion compared to the silver compositions.
The invention possesses a high degree of mechanical robustness or durability as characterized by abrasion resistance (using pencil hardness test) as well as a high degree of flexibility resulting in reduced loss of conductivity (&lt;two times initial resistance) after creasing.